Let's first understand that AT&T wants you to give up that grandfathered plan. Very badly. So they are not going to do ANYTHING whatsoever to make it attractive or easy to stay with your plan.

Now that that's understood ...

Ask yourself how often you really NEED to do FaceTime when there's no Wi-Fi nearby. Maybe your circumstances are different, but where I live (in a modest-sized though capital city) there's barely any need for cellular data at all! Nearly everywhere I go we have Wi-Fi; my house, friends' house, every coffeeshop and cafe, all of the downtown core, the mall, etc.

On top of that, FT burns through data like almost nothing else, so unless your plan was incredibly generous it would again be an option you'd only want to use sparingly if at all.

Only you know your situation but if there's Wi-Fi nearby where you usually are, I'd suggest sticking with the grandfathered plan and saving FaceTime for Wi-Fi only (better quality anyway, unless you'e on LTE).